José Holguín-Veras

Modeling commercial vehicle empty trips with a first order trip chain model

Abstract In this paper, new mathematical formulations that depict the flow of empty commercial vehicles as a function of a given matrix of commodity flows were developed. These formulations are based on probability principles and spatial interaction concepts. The models are based on the concept of order of a trip chain , defined as the number of additional stops with respect to the primary trip, and provide a statistical link between the first order and higher order trip chains. Three different destination choice probability functions were hypothesized based on different assumptions about the destination choice process. One of these formulations included a memory component, that takes into account the amount of travel already done in the destination choice process. An example, based on data from an origin–destination study in Guatemala City, is included to show the practicality of the proposed models. The numerical results indicated a slight superiority of the formulation that takes into account the length of the previous trip. In all cases, this model outperformed the previous models which seems to be an indication of the reasonableness of its fundamental assumptions and specifically of the benefits of including a memory function. The paper also provides empirical evidence of the importance of modeling empty trips. The root mean squared error of the estimation increased between 57% and 83%, with respect to the best empty trip model, if empty trips are not explicitly modeled.

Emergency Logistics Issues Affecting the Response to Katrina: A Synthesis and Preliminary Suggestions for Improvement

Extreme events pose serious logistical challenges to emergency and aid organizations active in preparation, response, and recovery operations, because the disturbances they produce have the potential to turn normal conditions suddenly into chaos. Under these conditions, delivering critical supplies (e.g., food, water, medical supplies) becomes an extremely difficult task because of the severe damage to the physical and virtual infrastructure and the limited or nonexistent transportation capacity. In this context, the recovery process is made more difficult by the prevailing lack of knowledge about the nature and challenges of emergency supply chains. As a result, the design of reliable emergency logistics systems is hampered by lack of knowledge about how formal and informal (emergent) supply chains operate and interact; methods to analyze and coordinate the flows of priority and nonpriority goods; and, in general, scientific methods to analyze logistics systems under extreme conditions. This paper describes the key logistical issues that plagued the response to Hurricane Katrina. The logistical failures following Katrina, which in August 2005 devastated the U.S. Gulf Coast, provide an example of the need to improve the efficiency of supply chains to the site of an extreme event. The paper is based on public accounts of the event and interviews conducted during a number of field visits to the Katrina-affected area in the aftermath of the event, as part of a research project funded by the National Science Foundation.

Overall impacts of off-hour delivery programs in New York city metropolitan area

This paper examines the chief findings of research conducted on policies to foster off-hour deliveries (OHDs) in the New York City metropolitan area. The goal was to estimate the overall impacts of eventual full implementation of an OHD program. As part of the research, a system of incentives was designed for the receivers of deliveries the system combined Global Positioning System (GPS) remote sensing monitoring with GPS-enabled smart phones to induce a shift of deliveries to the off-hours from 7:00 p.m. to 6:00 a.m. The concept was pilot tested in Manhattan by 33 companies that switched delivery operations to the off-hours for a period of 1 month. At the in-depth interviews conducted after the test, the participants reported being very satisfied with the experience. As an alternative to road pricing schemes that target freight carriers, this was the first real-life trial of the use of financial incentives to delivery receivers. The analyses indicate that the economic benefits of a full implementation of the OHD program are in the range of

Observed Trip Chain Behavior of Commercial Vehicles

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Optimal pricing for priority service and space allocation in container ports

193 million per year, corresponding to savings on travel time and environmental pollution for regular-hour traffic as well as productivity increases for the freight industry. The pilot test also highlighted the great potential of unassisted OHD—that is, OHD made without personnel from the receiving establishment present—because almost all participants who used this modality decided to continue receiving OHD even after the financial incentive ended.

Freight Generation, Freight Trip Generation, and Perils of Using Constant Trip Rates

Results are presented from a comprehensive analysis of the observed trip chain behavior of commercial vehicles in the Denver, Colorado, region on the basis of data collected by the Denver Regional Council of Governments. These analyses may provide the foundation for further research into commercial vehicle trip chain models. Trip chain behavior is characterized by number of trip chains, length of trip chains, and conditional probability of trip purpose. For this analysis, trip purposes were grouped as freight transportation; transport of people; service calls; fuel, service, and so forth; and return to base or home. Commercial vehicles were grouped as automobile, pickup van, single-unit truck, combination truck, and sports utility vehicle. The conditional probabilities were analyzed for a fixed length of trip chain and were conditioned on stop number. Although most vehicles were found to make one trip chain per day, one of four commercial vehicles made more than one trip chain per day. The analyses also i...

Material Convergence: Important and Understudied Disaster Phenomenon

This paper focuses on the determination of optimal space allocation and optimal pricing for priority systems in container ports. The problem is formulated taking into account the intrinsic and logistic cargo value, and a capacity constraint that considers the various physical requirements of the containers. Prices and space allocations are found for various cases, showing explicitly the role of each element. The resulting models extend classical price differentiation theory, i.e. the inverse elasticity rule, in various directions. Finally, the implications of these results and the corresponding information requirements are clearly established.

Trip Length Distributions in Commodity-Based and Trip-Based Freight Demand Modeling: Investigation of Relationships

Several findings call into question current practices. The chief conclusion is that the accuracy of freight generation (FG) and freight trip generation (FTG) models depends on the consistency between the models structure and actual FG-FTG patterns, the degree of internal heterogeneity of the economic and land use aggregation used to estimate the model, and the appropriateness of the spatial aggregation procedure used to obtain the desired FG-FTG estimates. Relative to model structure, the paper establishes strong reasons to treat FG and FTG as separate concepts, because the latter is the output of logistic decisions, whereas the former is determined by the economics of production and consumption. The connection between business size variables–for example, employment–and FG is relatively strong because they are economic input factors, whereas the one with FTG is weaker because inventory and transportation costs come into play. Thus it is generally not correct to assume proportionality between FTG and business size or to assume that using constant FTG rates could be problematic. For instance, only 18% of the industry sectors in New York City exhibit constant FTG rates per employee. For economic and land use aggregation, the finer the level of detail the better, as independent variables have a better chance to explain FG-FTG. In the case of spatial aggregation, the correct aggregation procedure depends on the underlying disaggregate model. For a FG-FTG model to work well, both economic and land use and spatial aggregations must be appropriate.

Parking in the City: Challenges for Freight Traffic

AbstractThis paper reports the research conducted on material convergence, which is one of the most important and, ironically, one of the most understudied disaster phenomena. This spontaneous flow of supplies, equipment, and general donations to the impacted area brings much-needed relief and major complications to the operations. The paper reviews empirical evidence from disaster literature and complements it with lessons learned from fieldwork to identify the problems created by the nonpriority component of the material convergence. The paper ends with policy suggestions regarding the use of appropriate material convergence management and control strategies.

A framework for an integrative freight market simulation

Commodity-based and vehicle-trip-based freight demand modeling is discussed. The characteristics of the trip length distributions (TLDs) are examined, defined in terms of tons, as required in commodity-based modeling, and in vehicle trips, as required in trip-based modeling. With data used from a major transportation study in Guatemala, the TLDs are estimated for both tons and vehicle trips. The analysis revealed that (a) the shape of the TLDs depends upon the type of movements being considered; (b) TLDs defined in terms of tonnage differ significantly from those defined in terms of vehicle trips; (c) TLDs for different types of vehicles, transporting similar commodities, reflect the range of use of each type of vehicle; (d) though tons TLDs and vehicle TLDs are different, the relationship between them seems to follow a systematic pattern that, if successfully identified, would enable transportation planners to estimate one type of TLD given the other; and (e) major freight generators affect the shape of the TLDs, so complementary models may be needed to provide meaningful depictions of freight movements.